Safety trocar

ABSTRACT

A safety trocar is provided in which the cannula is reciprocally mounted to the cannula housing and reciprocates into the cannula housing when a proximally directed force is applied thereto. The piercing tip is maintained in the exposed position by a latching mechanism associated with the obturator shaft, and is automatically withdrawn into the cannula under the force of a spring when the proximal force is removed from the cannula, the cannula thus advancing distally and releasing the latching mechanism. Penetration force is maintained at a minimum, and safe and efficacious trocar entry is facilitated.

TECHNICAL FIELD

This invention relates to a surgical trocar and more particularly to asafety trocar in which the sharp cutting tip retracts into the cannulaso as to minimize the likelihood of inadvertent injury to viscera andother internal tissue.

DESCRIPTION OF THE PRIOR ART

Trocars are sharp pointed surgical instruments used to puncture a bodycavity. Trocars are generally adapted to be used together with a tubulartrocar sleeve or cannula. Once the body cavity has been punctured by thetrocar, the sharp trocar is removed from the cannula, thereby leavingthe cannula extending into the body cavity. Endoscopic surgicalprocedures are then performed through the cannula with accessoryinstrumentation such as laparoscopes, dissectors, graspers, etc.

Commercially available safety trocars include a spring-loaded safetyshield which is adapted to cover the trocars tip once the body cavityhas been entered so as to provide an increased level of protection tointernal structures from puncture of laceration. For example, U.S. Pat.No. 4,601,710 to Moll describes a trocar assembly which consists of twosubassemblies: a trocar subassembly which includes a sharp-tipped trocarand a spring-loaded tubular safety shield positioned therearound, and acannula subassembly.

When ready for use, the trocar and safety shield of the trocarsubassembly are inserted through the cannula. The safety shield isinitially in its distal-most position covering the trocar tip. Exertionof pressure against the skin with the trocar causes the shield to bepushed rearwardly against the spring to expose the piercing tip of thetrocar. The tip penetrates the skin and underlying tissue with continuedpressure. Once the tip has penetrated through the wall and has enteredthe cavity, the force against the front end of the shield ceases and theshield is automatically moved back to its distally extended position.Viscera and other internal tissue are thus protected from contact withthe sharp piercing tip and potential damage therefrom.

An article entitled "Needle for the Puncture and Lavage of the AbdominalCavity" authored by F. S. Subairov discloses a safety device forpuncturing the abdominal cavity which consists of a hollow tube, astylet and a spring. The spring is soldered to the stylet and threadedinto the rear of the hollow tube. The distal end of the stylet isexposed from the hollow tube by pressing the stylet toward the tube,thereby compressing the spring. Once the stylet and tube enter a bodycavity, the tube is advanced under spring force to cover the distal endof the stylet. A similar device is disclosed in EP 350,291 (see FIGS.1-4).

U.S. Pat. No. 4,535,773 to Yoon suggests several alternative safetytrocar designs. In one embodiment (see FIGS. 22-28), a spring-loadedblunt probe is provided within the trocar shaft, as with conventionalVerres needles. The blunt probe is adapted to reciprocally slide throughan aperture in the trocar tip such that when the trocar tip enters abody cavity, the blunt probe springs distally forward through theaperture to prevent contact between the trocar tip and body organs. In asecond embodiment (see FIGS. 33-36), pressure sensors or transducers arefitted into the trocar blade surfaces and the distal end of the cannula.Sets of electrical leads run through the trocar shaft and communicatewith an alarm network in the proximal portion of the device. A furthermodification is suggested in which the trocar shaft is initiallymanually extended and maintained in its extended position by a detentwhich protrudes through a hole in the surrounding tubular structure. Thehole aligns with a solenoid socket. When the instrument is fullyassembled and the trocar tip is forced through a body wall, theelectrical leads running through the trocar shaft send electricalsignals to the solenoid which, at the appropriate instant, forces thedetent from the hole, allowing the trocar tip to withdraw into thecannula. Additional mechanisms for effecting withdrawal of cuttingimplements are also known. See, e.g., U.S. Pat. Nos. 4,375,815 to Burns;3,657,812 to Lee; and 3,030,959 to Grunert.

SUMMARY OF THE INVENTION

It has now been found that an improved safety trocar may be providedwhich includes:

(a) a cannula assembly comprising a cannula and a cannula housing;

(b) a trocar assembly comprising a sharp trocar tip, an obturator shaft,and a trocar housing;

(c) means associated with the obturator shaft which releasably maintainsthe trocar tip in an extended position;

(d) means associated with the cannula assembly for releasing thereleasable obturator means; and

(e) biasing means for retracting the trocar tip from the extendedposition to a retracted position in response to release of thereleasable obturator means.

The safety trocar of the present invention is adapted to be armed by thesurgeon immediately prior to use. Arming may be accomplished byadvancing a button which extends through the trocar housing, bycompressing the trocar housing toward the cannula housing, or by likemeans. Once armed, the trocar tip releasably protrudes beyond the distalend of the cannula.

As the surgeon presses the trocar, and more particularly the trocar tip,against the body wall of a patient, an incision into and through thebody wall is begun. With continued pressure by the surgeon, the distalend of the cannula comes into contact with the body wall. The initialcounterforce exerted by the body wall against the cannula causes amechanism associated with the cannula to position the obturator shaft(together with the cutting tip) for immediate retraction upon entry ofdistal end of the cannula into the body cavity. Thus, removal of thecounterforce from the distal end of the cannula, e.g., upon entering thebody cavity, results in immediate and automatic withdrawal of the trocartip into the cannula under the force of a biasing means, e.g., a spring.

In a preferred embodiment of the trocar, a latch is associated with theobturator shaft to which the trocar tip is mounted, the latch beingbiased radially outward and being adapted to engage an internal shelfformed in the cannula when the trocar is armed. The cannula isreciprocally mounted to the cannula housing and biased, e.g., by acompression spring, distally relative to the cannula housing. As thetrocar tip enters the body cavity and the body wall exerts force on thedistal end of the cannula, the cannula reciprocates proximally into thecannula housing. This cannula reciprocation repositions the cannula'sinternal shelf relative to the latch such that, upon distal movement ofthe cannula upon entry into the body cavity, the latch is released fromengagement with the internal shelf. A spring which was loaded uponarming the trocar is thus free to immediately retract the trocar tipinto the cannula. In a particularly preferred trocar embodiment,abutment means are provided on the exterior of the cannula toward itsdistal end to facilitate reciprocation thereof through contact with thebody wall.

The trocar of the invention is also designed to permit manual retractionor disarming of the cutting tip, if so desired. This is accomplished bymanually reciprocating the cannula housing and releasing, therebydisengaging the latch from the internal shelf. The trocar is alsotypically provided with an indicator which signals the surgeon as towhether the trocar is armed or disarmed. For example, the relativeposition of the button used to arm the trocar may be calibrated orindexed to communicate the trocar tip position or a window may beprovided through which a trocar tip position indicator is visible.

The trocar of the present invention provides a safe and efficaciousmeans for gaining access to body cavities to permit minimally-invasivediagnostic and surgical procedures to be accomplished. The trocar isequipped with a reliable mechanism for effectuating immediate, automaticretraction of the cutting tip into the cannula. Penetration force iskept to a minimum through the unique internal mechanism for releasablymaintaining the trocar tip in the armed position.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a side view of a trocar of the present invention in an armedconfiguration;

FIG. 2 is a top view of the trocar assembly of FIG. 1, partially incross section;

FIG. 3 is an exploded view of a spring assembly;

FIG. 4 is a front view of a top trocar housing;

FIGS. 5 and 6 are plan and side views, respectively, of aspring-retaining plate;

FIGS. 7 and 8 are partial side views of the cannula;

FIG. 9 is a plan view of a lower cannula housing;

FIG. 10 is a sectional side view of the cannula housing of FIG. 9 takenalong line 10--10;

FIG. 11 is a bottom view of a trocar housing;

FIG. 12 is a side view of a shelf insert;

FIG. 13 is a front view of the shelf insert of FIG. 12;

FIG. 14 is a side view of a latch;

FIG. 15 is a side view of a latch release finger;

FIG. 16 is a side view, partially in section, of a portion of a cannula;

FIG. 17 is a side view of a latch subassembly.

FIG. 18 is a top view, partially in section, of the proximal end of anunarmed trocar of the present invention; and

FIGS. 19-21 are top views, partially in section, of the proximal end ofan armed trocar of the present invention showing a sequence of positionswhich culminate in retraction of the trocar tip.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, trocar 10 is shown in its fully assembled conditionwith cutting tip 12 extending from cannula 14. Trocar 10 includes atrocar assembly 16 and a cannula assembly 18. The longitudinallyextending or endoscopic portion of trocar assembly 16 which extends fromtrocar housing 20 is shielded from view in FIG. 1 by cannula housing 21and cannula 14, except for extended cutting tip 12. A circumferentialabutment surface 15 is provided toward the distal end of cannula 14.

Referring now to FIGS. 2 and 3, pyramidal cutting tip 12 is rotatablymounted to obturator shaft 22 at swivel joint 24. A cylindrical channel26 is formed in the proximal end of obturator shaft 22. A tension spring28 (see FIGS. 1 and 3) is positioned within channel 26 and anchored toobturator shaft 22 at distal extension 30 by pin 32. An anchor pin 34which is adapted to mount to rear wall 36 of trocar housing 20 includesan aperture 35 which cooperates with proximal arm 37 to anchor tensionspring 28 to trocar housing 20.

A flange 38 is formed at the proximal end of obturator shaft 22encircling tension spring 28. Flange 38 forms a half-moon shaped button40 which slides within slot 42 in the upper face 44 of trocar housing20. Movement of button 40 within slot 42 moves obturator shaft 22 andcutting tip 12 relative to trocar housing 20. Tension spring 28 biasesobturator shaft 22 and its accompanying structure proximally, such thatflange 38 rests against rear face 46 of slot 42 absent externalinfluences.

Referring to FIG. 4, top housing 48 of trocar housing 20 is shown. Tophousing 48 includes mounting legs 50 for attachment to a bottom housing90 (FIG. 11). Upper face 44 includes a concave region 52 in the base ofwhich is formed slot 42. As most clearly seen in FIG. 1, button 40 issized and dimensioned to conform to and slide within concave region 52while extending slightly above upper face 44 of trocar housing 20. Thecooperation between concave region 52 and button 40 facilitatesunimpeded movement of button 40, particularly upon retraction of trocartip 12 as discussed below, without sacrificing convenient thumb accessto button 40 for arming of trocar 10 by the surgeon.

Turning to cannula assembly 18, cannula 14 defines a tubular lumen andis reciprocally mounted to cannula housing 21. Referring to FIGS. 2 and5-8, a spring-retaining plate 54 is mounted to flange 56 at the proximalend of cannula 14, e.g., by adhesive or welding, with central aperture58 in plate 54 aligned with the lumen through cannula 14. Aperture 58 issized to accommodate unencumbered passage of obturator shaft 22 andincludes oppositely directed extension arms 62, each arm 62 having anaperture 64. Apertures 64 receive and frictionally engage cylindricalpins 66. Aperture extension 60 is formed in one extension arm 62allowing passage of latch subassembly 147 therethrough, as describedbelow.

Referring again to FIG. 2, cannula housing 21 receives spring-retainingplate 54 with cylindrical pins 66 facing proximally. Compression springs68 are positioned against extension arms 62 and around pins 66. Pins 66thus act to position and support springs 68. Chambers 70 are formed incannula housing 21 to receive and capture the opposite ends ofcompression springs 68. Reciprocation of cannula 14 into cannula housing21 causes proximal movement of plate 54 which compresses springs 68within chambers 70, thereby biasing cannula 14 in the distal direction.

Returning to FIGS. 7 and 8, the proximal end of cannula 14 includes twoslots 72 and 74, preferably separated by at least 90°. Alignment slot 72serves to maintain rotational alignment of cannula 14 with respect tocannula housing 21. Referring additionally to FIGS. 9 and 10, alignmentpin 76 is fixedly secured within aperture 78 in lower cannula housing 80and, when fully inserted into housing 80, extends into lumen 82. Inassembling cannula 14 within lower cannula housing 80, alignment pin 76is positioned within alignment slot 72, thereby preventing rotation ofcannula 14 yet permitting axial movement of cannula 14 relative tocannula housing 21. Additionally, inwardly directed orientation pin 84on proximal housing extension face 86 cooperates with an orientationslot 88 in bottom housing 90 (see FIG. 11) to ensure proper alignmentbetween cannula housing 21 and trocar housing 20. A gasket 92 andstabilizer plate 96 are positioned within flange 94 in lower cannulahousing 80 to provide a gas seal with inserted instrumentation, and tocooperate with an internal flapper valve, as is known in the art.

Slot 74 in cannula 14 forms an internal shelf 98 with which latch 100(see FIG. 14) is adapted to engage. Internal shelf insert 102 (FIGS. 12and 13) is positional within cavity 104 in lower cannula housing. Shelfinsert 102 comprises angled latching faces 106, bridging arm 108 andlongitudinal slot 110. Shelf insert 102 is positioned within cavity 104such that angled latching faces 106 are directed distally and edges 112are substantially aligned with internal shelf 98 of cannula 14. Bridgingarm 108 abuts outer wall 114 of cavity 104 and is typically securedthereto, e.g., by an adhesive.

Latch 100 has a body 116 which forms a latch finger 118 which includesan outer camming face 120 and an inner latching face 122. Aperture 124is located in mid-region 117 of body 116 and permits latch 100 to bemovably joined to latch release finger 126 (see FIGS. 15 and 17).Mid-region 117 is of reduced thickness relative to the remainder of body116 to accommodate latch release finger 126. Aperture 128 is formed inthe region of body 116 distant from latch finger 118.

Latch release finger 126 includes a substantially triangularly-shapedextension 138 having distal and proximal cam faces 140 and 142,respectively. Latch release finger 126 also includes an aperture 144 anda spring abutment region 145. As shown in FIG. 16, latch release finger126 is movably mounted to latch 100 by means of a pin (not pictured)which passes through apertures 144 and 124 to form a latch subassembly147. When release finger 126 is positioned such that extension 138 isdirected toward aperture 128, abutment region 145 extends below latchwall 148.

Latch subassembly 147 is pivotally joined to obturator shaft 22 by a pin(not pictured) which passes through aperture 128 in latch body 116 andaperture 132 in hollow region 130 of shaft 22. As shown in FIG. 16, aleaf spring 134 is mounted to shelf 136 in hollow region 130, e.g., byan adhesive. The proximal region 146 of leaf spring 134 biases latch 100clockwise around aperture 128 and biases release finger 126counter-clockwise relative to aperture 144. Hollow region 130 is sizedand dimensioned to permit latch subassembly 147 to be fully recessedtherewithin (against the bias of leaf spring 134). Inasmuch as releasefinger 126 is restricted in its counter-clockwise rotation by latchfinger 118 and/or abutment with wall 119, and in its clockwise rotationby engagement between spring abutment region 145 and leaf spring 134,the full range of rotation of release finger 126 is approximately 90°.

In use, and referring generally to FIGS. 18-21 cutting tip 12 andobturator shaft 22 are introduced through cannula housing 21 and intocannula 14. When obturator shaft 22 is fully inserted, trocar housing 16abuts cannula housing 21. However, cutting tip 21 remains within cannula14 until trocar 10 is armed by the surgeon. To arm the instrument, thesurgeon advances button 40 within channel 42 which distally advancesobturator shaft 22 and causes cutting tip 12 to extend beyond cannula14. Distal movement of obturator shaft 22 also results in distalmovement of latch subassembly 147.

Leaf spring 134 biases latch subassembly 147 outward from hollow region130. As obturator shaft 22 moves distally, outwardly biased latchsubassembly 147 passes through aperture extension 60 in spring-retainingplate 54. Outer camming face 120 of latch 100 then contacts flange 56 atthe proximal end of cannula 14, causing counterclockwise rotation oflatch subassembly 147 relative to aperture 128 (see FIG. 17). Thiscounterclockwise rotation recesses latch subassembly 147 within hollowregion 130 against the bias of leaf spring 134.

As cutting tip 12 approaches its fully armed position, latch subassembly147 comes into alignment with slot 74 in cannula 14. Outer camming face120 is thus freed from contact with the inner surface of cannula 14, andleaf spring 134 causes latch subassembly 147 to rotate clockwise suchthat latch finger 118 extends radially outward from hollow region 130.However, latch release finger 126 remains positioned such that extension138 is directed substantially toward aperture 128, with abutment region145 resting against leaf spring 134. The freeing of camming face 120from contact with cannula 14 is generally accompanied by an audibleclick, signalling the surgeon that trocar 10 is armed and further distalmovement of button 40 is unnecessary.

When the surgeon releases button 40, tension spring 38 draws obturatorshaft 22 proximally until inner latching face 122 of latch 100 engagesinternal shelf 98 and shelf insert 102. Latch release finger 126 passesback within cannula 14 proximal of slot 74, thereby pressing abutmentregion 145 against the bias of leaf spring 134. In this fully armedposition of FIGS. 1 and 2, cutting tip 12 extends beyond cannula 14 andbutton 40 is distally located within slot 42.

The surgeon presses armed trocar 10 against the body wall of a patient,thus causing piercing tip 12 to incise the tissue. As cutting tip 12passes through the body wall, the distal end of cannula 14 is broughtinto engagement with tissue. The counterforce exerted by the body wallagainst cannula 14 and abutment surface 15 causes cannula 14 to moveproximally against the bias of compression springs 68. This proximalmovement of cannula 14 release latch release finger 126 to enter slot74. Latch release finger 126 rotates counterclockwise relative toaperture 144 within longitudinal slot 110 in shelf insert 102.

As the surgeon continues to press trocar 10 against the body wall,piercing tip 12 enters the body cavity. Continued pressure by thesurgeon advances cannula 14 into the body cavity as well. As soon as thecounterforce of the body wall against the distal end of the cannula 14and abutment surface 15 is surpassed by the distally-directed force ofcompression springs 68 against spring-retaining plate 54, cannula 14 isdriven distally relative to cannula housing 18. As cannula 14 movesdistally, internal shelf 98 contacts proximal cam face 142 of latchrelease finger 126. Latch release finger 126 is thus drivencounterclockwise such that extension 138 abuts latch finger 118 and/orthe body of latch release finger 126 abuts wall 119 of latch 100. Inthis position, latch release finger 126 prevents engagement betweeninternal shelf 98 and latch finger 118.

As cannula 14 continues to move distally, internal shelf 98 cams latchsubassembly 147 counterclockwise relative to aperture 128 against thebias of leaf spring 134, thus freeing latch finger 118 from engagementwith shelf insert 102. As soon as the tip of latch finger 118 rotatesout of engagement with shelf insert 102, tension spring 28 drawsobturator shaft 22 and cutting tip 12 proximally such that cutting tip12 is positioned within cannula 14. Button 40 is also drawn proximallywithin slot 42 and is once again positioned to allow the surgeon to armtrocar 10, if so desired.

If, after arming trocar 10, the surgeon determines that it is desirableto manually retract cutting tip 12 into cannula 14, the surgeon simplymoves cannula 14 proximally with respect to cannula housing 18 andreleases. Cannula 14 will then move distally under the bias ofcompression springs 68 rotating latch subassembly 147 counterclockwise.Latch finger 118 is thus moved out of engagement with shelf insert 102,allowing tension spring 28 to withdraw cutting tip 12 into cannula 14.

The position of button 40 within slot 42 provides the surgeon with avisual indication of the position of cutting tip 12 relative to cannula14. The audible click associated with the movement of latch 100 duringthe arming of trocar 10 also provides an aural signal to the surgeon.

Many structures may be included toward the distal end of cannula 14 tofacilitate the sensing of body wall counterforce to effectuatereciprocation of cannula 14, abutment surface 15 being but one example.Exemplary structures include radially spaced, outwardly directedprotuberances, inflatable means of the type known in the art for fixedlypositioning catheters and like devices, and flange means of varyinggeometries. Such structures may be fixedly secured to cannula 14 orrepositionable along the longitudinal axis of cannula based on suchfactors as patient size and weight.

While the above description contains many specific details, thesedetails should not be construed as limitations on the scope of theinvention, but merely as examples of preferred embodiments thereof.Those skilled in the art will envision many other possible variationsthat are within the scope and spirit of the invention as defined by theclaims appended hereto.

I claim:
 1. A trocar comprising:(a) a cannula assembly comprising acannula and a cannula housing defining an internal cannula passage, saidcannula being reciprocally mounted with respect to said cannula housingfor movement between a distal position and a proximal position; (b) atrocar assembly adapted to cooperate with said cannula assembly, saidtrocar assembly comprising a piercing tip mounted to an obturator shaftand a trocar housing, said piercing tip and at least a portion of saidobturator shaft being configured and dimensioned for entry into saidinternal cannula passage; (c) releasable obturator means associated withthe obturator shaft which releasably maintains the piercing tip in anextended position relative to said cannula; (d) release means associatedat least in part with the cannula assembly and actuated by movement ofsaid cannula with respect to said cannula housing for releasing thereleasable obturator means; and (e) biasing means for retracting saidpiercing tip and said obturator shaft from said extended position to aretracted position in response to release of the releasable obturatormeans.
 2. The trocar of claim 1, wherein said cannula includes a slotwhich defines an internal shelf and said internal shelf is adapted tocooperate with said releasable obturator means to maintain said piercingtip in said extended position.
 3. The trocar of claim 1, wherein saidreleasable obturator means comprises a latch and a spring biasing saidlatch radially outward.
 4. The trocar of claim 1, wherein said releasemeans for releasing said releasable obturator means comprises a slot insaid cannula.
 5. The trocar of claim 4, wherein said release meansfurther comprises a latch release finger associated with said releasableobturator means and said slot in said cannula is adapted to align withsaid latch release finger.
 6. the trocar of claim 1, wherein said trocarassembly further comprises an indicator means which signals the positionof said piercing tip.
 7. The trocar of claim 1, wherein said releasemeans for releasing the releasable obturator means and said biasingmeans cooperate to automatically retract said piercing tip to saidretracted position in response to removal of a counterforce from thedistal end of said cannula.
 8. The trocar of claim 1, wherein saidcannula assembly further comprises cannula biasing means.
 9. The trocarof claim 8, wherein said cannula biasing means comprises at least onespring biasing said cannula toward said distal position.
 10. The trocarof claim 1, wherein said cannula includes an abutment means.
 11. Thetrocar of claim 10, wherein said abutment means is positioned on theexterior of said cannula toward the distal end thereof.
 12. The trocarof claim 10, wherein said abutment means is selected from the groupconsisting of a circumferential abutment surface, a plurality ofradially spaced, outwardly directed protuberances, inflatable means, andflange means.
 13. The trocar of claim 10, wherein said abutment means isrepositionable along the longitudinal axis of said cannula.
 14. Thetrocar of claim 1, wherein said releasable obturator means comprises alatch and a leaf spring.
 15. The trocar of claim 14, wherein saidrelease means comprises a latch release finger and said latch isrotatably joined to said latch release finger by pin means.
 16. Thetrocar of claim 1, wherein said obturator shaft includes a hollow regionin which is mounted a leaf spring and a latch subassembly, said leafspring biasing said latch subassembly radially outward.
 17. The trocarof claim 1, wherein said release means for releasing said releasableobturator means comprises a shelf insert, said shelf insert forming alongitudinal slot.
 18. A trocar comprising:(a) a cannula assemblycomprising a cannula reciprocally mounted to a cannula housing, saidcannula defining an internal shelf; (b) a trocar assembly comprising apiercing tip, an obturator shaft and a trocar housing; (c) latch meansassociated with said obturator shaft, said latch means adapted toreleasably engage said internal shelf; and (d) biasing means forretracting said piercing tip from an extended position wherein saidpiercing tip extends beyond said cannula to a retracted position whereinsaid piercing tip is shielded by said cannula; wherein reciprocation ofsaid cannula with respect to said trocar assembly releases said latchmeans from engagement with said internal shelf and allows said biasingmeans to retract said piercing tip to said retracted position.
 19. Thetrocar of claim 18, wherein said latch means comprises a latch and aleaf spring abutting said latch.
 20. The trocar of claim 18, whereinsaid cannula assembly further comprises a shelf insert positionedadjacent said internal shelf, said latch means engaging both saidinternal shelf and said shelf insert.
 21. The trocar of claim 18,wherein said cannula may be reciprocated manually by a surgeon orthrough contact with a body wall of a patient.
 22. The trocar of claim11, wherein said cannula assembly further comprises abutment means onsaid cannula.
 23. A method for inserting a trocar, comprising:(a)advancing an obturator shaft and a piercing tip to expose said piercingtip from a cannula, said piercing tip being maintained in said exposedposition through engagement of latch means associated with saidobturator shaft with a shelf formed in said cannula; (b) pressing saidpiercing tip against a body wall, said body wall thereby exerting acounterforce against said cannula and causing reciprocation of saidcannula into a cannula housing against the bias of cannula biasingmeans; (c) entering said piercing tip into a body cavity, therebyremoving said counterforce from said cannula, said cannula biasing meanscausing said cannula to move distally, thereby releasing said latchmeans from engagement with said shelf, whereby said piercing tip isautomatically withdrawn into said cannula under the action of obturatorbiasing means.
 24. The method of claim 23, wherein said latch meanscomprises a latch and a leaf spring abutting said latch and biasing saidlatch radially outward.
 25. The method of claim 23, wherein said cannulaincludes abutment means which facilitate sensing of counterforce by saidcannula.
 26. A trocar comprising:(a) a cannula assembly comprising acannula and a cannula housing defining an internal cannula passage; (b)a trocar assembly adapted to cooperate with said cannula assembly, saidtrocar assembly comprising a trocar housing and obturator means whichincludes a piercing tip at a distal end thereof, at least a portion ofsaid obturator means being configured and dimensioned for entry intosaid internal cannula passage; (c) button means mounted to saidobturator means and extending from said trocar housing for advancingsaid piercing tip to an extended position relative to said cannula; (d)latching means mounted to said trocar assembly and adapted to cooperatewith engagement means, said latching means adapted to assume a latchedposition wherein said latching means engages said engagement means andmaintains said piercing tip in said extended position relative to saidcannula, and an unlatched position wherein said latching means isreleased from engagement with said engagement means; and (e) biasingmeans for retracting said piercing tip from said extended position to aretracted position after said latching means has assumed said unlatchedposition.
 27. The trocar of claim 26, wherein said engagement meanscomprises a shelf formed on said cannula.
 28. The trocar of claim 26,wherein said latching means is mounted to said obturator means.